In the production of pressed board it is known to provide platen presses or the like in which a mat of a material capable of fusion to form the pressed board is compressed between two planar surfaces, generally forming the bedplate and headplate of a platen press.
Typical of the systems provided for this purpose is that of my U.S. Pat. No. 3,860,381 and the earlier systems described or referenced in that patent or in the file thereof.
The term "pressed board" is used herein in its most generic sense to identify hardboard, i.e. material of high structural strength, low fluid permeability and excellent surface characteristics, as well as to less dense and highly porous boards which may be used as thermal or acoustical insulation, for interior finishing, or the like. The term is generic to any relatively rigid generally flat board structure, formed by compression utilizing both heat and pressure, to bond fibers, chips or dust of wood or some other material, e.g. a cellulosic material, with intrinsic thermally activatable binder or an adder binder, the board being pressed from a structure which, for convenience, will be defined as a mat.
The term "mat" is used herein likewise in its most generic and general sense to refer to a layer of more or less coherently interrelated particles and the binder.
The aforementioned publications describe how such materials may be introduced into a platen press and how the finished board may be carried away for further processing.
It is known to provide a platen press for the compaction of particleboard mats, i.e. for the production of chipboard, fiberboard and sawdust board, which comprises a press frame or support structure, a fixed press plate or bed, a movable press plate or bed, and a multiplicity of working cylinder arrangements which are disposed symmetrically with respect to a vertical median longitudinal plane through the press. The working cylinder arrangements will each be understood to include at least one cylinder and at least one piston, either the cylinder or the piston being connected to the press support or frame while the other of these members is connected to the movable press plate.
Such systems have also been provided heretofore with a plurality of auxiliary piston-and-cylinder arrangements which can be used to modify the orientation of the movable press member so as to ensure generally parallel pressing of the board. Naturally, control systems can be provided for the hydraulic piston-and-cylinder arrangements.
The movable and stationary press beds are generally formed with pressing plates which can be heated, e.g. electrically or by fluid heating techniques, the movable and/or stationary beds each including, in addition to the respective press plate, a plurality of beams which generally extend transversely of the longitudinal axis of the press and serve as points of attack for the hydraulic cylinder arrangements.
Naturally, for high quality pressed board, it is desirable that the compression be of the parallel type. The term "parallel compression" is used herein to indicate that the upper and lower faces of the pressed board, at least as the latter emerges from the press, are as close to parallel to one another as possible. Since these surfaces are also usually planar, reference may also be made to plane-parallel surfaces.
Plane-parallel surfaces of a pressed board indicate that neither surface has bulges which can be considered as deviations from planarity, furthermore the parallel requirement specifies that the board is of uniform thickness throughout its length and breadth.
Obviously, for plane parallel compression of the pressed-material mat, it is essential to compensate for any elastic deformation of the pressing beams, the beds or the pressing plates.
A pressing frame, in accordance with the present description, is intended to include any type of press support commonly in use. For example, the press support may be a post structure carrying the bed and upon which the upper member of the press is mounted, e.g. a headplate, or a row of portal frames spaced apart in the longitudinal dimension of the press. A system of the latter type is shown for the platen press of U.S. Pat. No. 3,860,381.
The movable press member or bed can be either the upper or the lower bed.
The movable press bed is generally lighter than the fixed press bed and, when the deformation of the press beds is considered, is the more readily deformed member.
If parallel compression of the mat and the boards is desired without compensation by the prior-art techniques, one must either resort to extraordinarily massive press beds, incapable of any deformation during the pressing operation, or must accept a certain degree of convex or concave curvature resulting from the distortion of the movable press bed.
It is possible, following the pressing operation, to remove the convex or concave curvature in the pressed board by expensive and time-consuming shaping, material-removal or like aftertreatments.
The principles of the present invention, as will be described hereinafter, are applicable to the single-stage or single-level platen presses of the type described in the aforementioned patent as well as to double-level or two-stage presses as likewise have been illustrated therein. In the latter case, the fixed press bed is the intermediate bed, the upper and lower plates being movable. The invention is also applicable to multilevel presses in which the press plates between the stages are movable. In this case, of course, the parallel compression must be dealt with as a sum so that the individual distortions of the intervening plates as well as the upper or lower movable plate can be compensated. Finally, the system of the present invention will be understood to be applicable as well to the compression of asbestos cement plates or boards and the like.
The prior-art apparatus, represented for example in German patent document (open application--Offenlegungsschrift) DE-OS No. 15,02,042, is shown as a single-level press. In this apparatus, the working piston-and-cylinder arrangements are disposed outwardly, as seen in plan view, of the longitudinal median plane of the press and their respective points of attack lie inwardly of the spacer bars which are provided between the press plates to establish the thickness of the pressed board to be formed in the press.
The auxiliary piston-and-cylinder arrangements, provided primarily to equalize or compensate for distortion of the movable press plate, are disposed generally in the region of the longitudinal vertical median plane of the press. These auxiliary cylinders thus act as compensating cylinder arrangements which are intended to level the aforementioned deformations of the movable press member under the control of a control arrangement or device.
The movable press member, in the operation of the conventional apparatus, is initially displaced by actuating all or individual ones of the compensating cylinder arrangements downwardly until the press plate comes to lie on the mat to be compressed.
During this step, the working cylinders are merely entrained. The respective pressurizable compartments behind the pistons of these cylinders are, of course, filled with fluid, e.g. from an accumulator or storage vessel, although an elevated pressure is not generated in these compartments during the entrainment phase.
When the press plate contacts the mat, the working cylinders are actuated so that their pistons apply the full pressure to the plate and are driven by the full hydraulic pressure in the compression mode. This, of course, forces the upper or movable press plate downwardly to compact the mat.
As already implied, the working cylinders are disposed relatively far from the spacer bars. As a consequence, upon compression of the mat, depending upon the nature of the material pressed and the height of the layer forming the mat, the movable press plate deforms or bulges to a greater or lesser extent together with the pressing beam.
When the pressing plate of the movable bed engages the spacer bars, the pressure in the working cylinders increases markedly.
Pressure-sensing devices respond to this pressure increase and generate a control pulse which cuts off the main cylinders and, in some cases, relieves them. By contrast, the compensating cylinders generally remain under the working pressure to compensate for the bulging or deformation of the movable press bed.
If the bulge is in the direction of the fixed bed, the corresponding compensating cylinder is relieved. Conversely, if the bulge is in the direction opposite the fixed bed, the compensating cylinder may be additionally pressurized. Naturally, the compensating operations must be determined experimentally for each type of mat, for each board-producing program, for the various pressure levels and pressing periods operating in the press.
Even with these empirical derivations of the response to be made by the compensating cylinders, one cannot completely exclude significant variations resulting from changes in the nature of the mat or the like. The same applies for the significant variations in the counterpressure generated in the mat and opposing advance of the movable press plate during the pressing operation and resulting from premature hardening of the binder or variations in the moisture level in the particle material of the mat under the effect of heat from the heating plates.
As a consequence, the response of the movable press plate to the particular pressing operation cannot always be determined in advance and, even when determined in advance, is not necessarily constant or reproducible.
As a consequence, the control method of the prior art is extremely complicated and must permit relatively large tolerances. It is, therefore, practically impossible with the prior-art system to provide plane parallelity of the pressed boards with extremely narrow or small tolerances.
Another disadvantage of the prior-art system is that significant moments are generated between the main cylinders and the spacer bars in the press plates so that the press plates can impart to the boards a substantially corrugated surface which must be sanded down or otherwise treated. This is especially the case when the movable press bed is made relatively light to save material and hence is especially bendable.
Reference may be had to German patent document (printed application-Auslegeschrift) DE-AS No. 17,03,297 in which similar problems may arise although spacer bars are avoided and the working cylinders or main cylinders are provided at the outer edges of the movable press bed or plate.
Another system, represented by German patent document (utility model-Gebrauchsmuster) DE-GBM No. 19,24,142 in which the main cylinder arrangements are disposed along the edge of the movable press bed and the press plates are so arranged that these main cylinders lie directly above the spacer bars. This, however, precludes a proper compensating operation to overcome deformation of the movable press bed or plate.